1. Field of the Invention
The present invention relates to a light emitting apparatus, an exposure apparatus for carrying out exposure on a photosensitive body by use of the light emitting apparatus, and an image forming apparatus for forming an image by exposure on the photosensitive body. In the image forming apparatus of this type, the exposure is carried out on the photosensitive body by the electrophotographic method using the light emitting apparatus comprised of a plurality of light emitting elements, to form an image on this photosensitive body.
2. Related Background Art
The formation of an image by the electrophotographic method is a well known sequential process including steps of focusing an optical image from the light emitting apparatus on the photosensitive body to form a latent image thereon, developing the image with toner, transferring the image onto a transfer medium (paper), fixing the image thereon, and finally cleaning the photosensitive body.
In the image forming apparatus a laser optical system for scanning with a laser beam by use of a polygon mirror is commonly used as an exposure section for forming the latent image on the surface of the photosensitive body. The reason is the advantage of high resolution and high operation speed. The apparatus, however, needs to have a large space for placing such optical components as the polygon mirror, lenses, and so on, and it is thus not easy to downsize the structure of the apparatus. There is also a limit on how much the operation speed can be increased, because the scanning with the laser beam is effected by mechanical rotation of the polygon mirror.
Attention is thus being given to light emitting element arrays including a plurality of organic light emitting elements free of the above problems, as the light emitting apparatus. With the light emitting element arrays, the above problems of the placement space of the laser optical system and the limit of the operation speed do not arise, because the scanning is carried out using electric signals without need for the mechanical scanning and because the optical components such as the polygon mirror, lenses, and so on are not necessitated. In a specific structure the plurality of light emitting elements are each made of a transparent anode layer, an organic compound layer, and a cathode layer successively formed on a transparent substrate of a long beltlike shape and are arranged in a linear form. The organic compound layer is formed in a long beltlike shape as a common layer to the plurality of light emitting elements and is sandwiched between the anode and the cathode of a predetermined pattern on the top and bottom sides, to emit light individually.
The organic compound, however, has such a property that it is weak against water. On the other hand, the operation of the light emitting elements is characterized in that they are energized irregularly and discontinuously according to input data. For example, there often appear periods of a waiting state without input of data and the like during the operation and the temperature is lowered during such periods. Particularly, when the apparatus acts under high humidity circumstances, water is condensed to form dews around the light emitting elements. When the light emitting elements include the organic compound layer, the dews promote degradation of the light emitting elements, so as to pose the problem of shortening their lifetime accordingly.